According to an international foam market survey, "Fire Fighting Foam Concentrates," by IMR International in 1992 the use of polar-solvent fire-fighting foam concentrates is becoming increasingly wide-spread in the fire-fighting industry throughout the world because of their effectiveness on various types of flammable solvents and fuels. This type of foam concentrate is also known in the fire-fighting industry as "alcohol-resistant aqueous film forming foam (AR-AFFF)," "alcohol-resistant concentrate (ARC)," "alcohol type concentrate (ATC)," or simply "multipurpose foam." The same survey reports that "alcohol-resistant AFFF is the most widely used type of concentrates in the USA in terms of value." Although these products are referred to as "polar-solvent" or "alcohol" resistant concentrates, they are generally designed to extinguish fires of both polar, hydrophilic liquids such as alcohols and nonpolar, hydrophobic fuels.
Polar-solvent fire-fighting foam concentrates generally contain as the key active components perfluoroalkyl surfactants (hereafter referred to as perfluorosurfactants), non-fluorosurfactants and water-soluble or swellable polymeric materials. They also often contain water-soluble solvents as foam stabilizers and other additives to accomplish specific tasks such as corrosion inhibition. The perfluorosurfactants, when used along with hydrocarbon surfactants, provide film-forming characteristics on the surface of a non-polar hydrophobic liquid. Fire-fighting foam concentrates whose performance depends only on their aqueous film-forming properties and which are useful on nonpolar liquids and fuels are referred to as AFFF (Aqueous Film Forming Foam) concentrates. U.S. Pat. No. 5,085,786 describes the state of the art and various perfluorosurfactants and non-fluorosurfactants typically used in the formulation of AFFFs. The use of AFFF agents, however, is limited to only non-polar liquid fires. They are ineffective on polar liquids which are either completely or partially miscible with water because the foam becomes destabilized when it comes in contact with such liquids. This is why polar-solvent fire-fighting foam concentrates also generally contain as a key component polymeric materials, typically polysaccharides.
Chiesa et. al. in their pioneering patents, for example, U.S. Pat. No. 4,060,489, disclose that water soluble (swellable) thixotropic polysaccharide "gums" cause the aqueous foam to form a gelatinous "mat" or "membrane" when it comes in contact with a polar, hydrophilic liquid. The formation of the membrane is caused by the fact that the polysaccharides are generally insoluble in flammable polar, hydrophilic liquids such as alcohols and ketones. The lack of solubility causes the hydrated (swollen) polysaccharide molecules to precipitate out of the foam, which eventually results in the formation of a floating gelatinous membrane at the interface between the foam and the polar liquid. This membrane acts as a diffusion barrier toward the fuel liquid and vapor and protects the rest of the foam mass from breaking. Hence it greatly increases the foam stability and consequently the fire-extinguishing effectiveness. The fire-fighting effectiveness of this membrane depends mainly on its thickness and the degree of hydration or swelling of the polysaccharide gums used.
U.S. Pat. Nos. 3,957,657; 4,060,132; 4,060,489; 4,149,599; 4,306,979; 4,387,032; 4,420,434; 4,464,267; 4,859,349 and 4,999,119 disclose polar-solvent fire-fighting compositions containing perfluorosurfactants, non-fluorosurfactants and polysaccharide gums.
The polysaccharide gums commonly used in commercially available polar-solvent foam concentrates are various grades of xanthan gums, the trade names of which include Rhodopol, Kelco, Keltrol, such as Keltrol BT and Keltrol RD, Actigum, Kelzan, Cecalgum and Galaxy, to name a few. Kelco K8A13, which is a special blend of polysaccharides is also commonly used. Other types of gums useful for the practice of this invention include rhamsan gum, such as Kelco K1A112 and welan gum. Kelco products are manufactured by Kelco, a Merck company.
Polar-solvent fire-fighting foam concentrates are typically formulated to be diluted (proportioned) to different concentrations for use on different types of fires. For fires involving nonpolar fuels such as hydrocarbon liquids, the concentrates are proportioned at the time of application through the foam nozzle to the 3% level (3 parts concentrate with 97 parts fresh or salt water). Fires involving polar solvents require a dilution to 6% (6 parts concentrate and 94 parts water). These products are known in the industry as "3.times.6" ("three by six") products.
Recent advances in polar-solvent fire-fighting formulation technology have made it possible to formulate concentrates that can be diluted at a single proportioning level of 3% for all uses. These products are known as "3.times.3" ("three by three") products. They require nearly twice as much polysaccharide gum and other active ingredients as the 3.times.6 formulations in order to provide the same fire extinguishing performance on polar solvents. Among the well known advantages of these new 3.times.3 products are savings in storage space, and savings in cost through the reduced usage level of the concentrate. These products can also eliminate the need on the part of the fire fighters to identify the fuel type in emergency situations, because only one, single proportioning setting at 3% is required for either polar solvent or hydrocarbon fires. With the 3.times.6 type of products, the fuel type, i.e. whether the burning fuel is a polar, hydrophilic liquid or a hydrocarbon, has to be identified before the fire fighter can properly set the foam proportioning device at either 6% or 3%.
One of the most serious disadvantages of the 3.times.3 type of polar-solvent compositions is their extremely high viscosities. These high viscosities, which range from 4000 to 5500 cP in the products currently available on the market, are caused by the high level of polysaccharide gums, typically 1.2-2.0% in the products. In fact, those skilled in the art of formulating polar-solvent fire-fighting concentrates using polysaccharide gums would like to be able to use more of the gums to improve the fire-fighting performance. Some polysaccharide gums have excellent hydration properties, which are important for fire-fighting performance as mentioned earlier, but because of the impractically high viscosities of formulations containing them, one is forced to choose either less hydrating gums or use decreased amounts, thus compromising the quality of fire-fighting performance.
Low viscosity products would provide the following advantages:
Easier handling of pumping, eduction and transport of the concentrate, especially during the winter time when the viscosity build-up at low temperature can lead to a gelled condition. Gelled concentrates are not usable. PA1 Rapid dilution and thereby accurate proportioning of the concentrate. If the viscosity is sufficiently low (to be considered as nearly a Newtonian system), this type of polar-solvent foam concentrate can be handled like AFFF concentrates, i.e. it can be directly added to the water (known as a "direct dumping" method in the fire fighting industry) without the use of an eductor. PA1 No requirement for additives to speed up the dissolution of polysaccharide gums in water: For example, Chiesa (U.S. Pat. No. 4,149,599) suggested using urea for this purpose. PA1 Freeze-protected products can be formulated. Due to the extremely high viscosity at freezing temperatures, current products cannot be formulated as such. PA1 a. one or more anionic perfluorosurfactants, 0.5-3.0% by weight, preferably 1.0-2.0%, PA1 b. one or more cationic perfluorosurfactants, 0. 1-2.0% by weight, preferably 0.5-1.2%, PA1 c. one or more non-ionic or amphoteric perfluorosurfactants, 1.0-4.0% by weight, preferably 2.0-4.0%, PA1 d. one or more non-fluorosurfactants, 8.0 to 25.0% by weight, preferably 18.0-22.0%, PA1 e. a perfluoroalkyl-terminated co-oligomer of acrylamide and acrylic acid, preferably LODYNE K90'90, which is available from Ciba-Geigy Corporation, 0-6.0% by weight, preferably 2.5-4.0%, PA1 f. a water miscible organic solvent as foam booster, 2.0-15.0% by weight, preferably 5.0-10.0%, PA1 g. one or more polysaccharide gums, 1.0-2.0% by weight, preferably 1.2-1.7%, PA1 h. a water-soluble, anionic copolymer of the present invention, 0.5-3.5% (actives) by weight, preferably 1.0-2.0%, and PA1 i. water, 20-50% or in the amount to make up the balance of 100%.
Only a few documented efforts or attempts have been found in the literature to reduce the viscosity of polysaccharide-containing polar-solvent fire-fighting foam concentrates. U.S. Pat. Nos. 4,060,489 and 4,387,032, for example, disclose the use of urea, thiourea, or certain inorganic salts to reduce the viscosity. No teachings have been found on the subject of reducing the viscosity of the 3.times.3 type of products. Several methods to reduce the viscosity have been found, all based on the control of the degree of swelling of the polysaccharide gum. Inorganic salts such as sodium or ammonium chloride or acetate, for example, were found to limit the swelling of the gum in a typical polar-solvent fire-fighting concentrate. However, the degree of viscosity reduction by this method is not as large as by the method of this invention. Certain solvents such as butyl carbitol can also be used to limit the swelling of a gum, and thus reduce the solution viscosity. This method requires a large amount of the solvent to reduce the viscosity significantly, which is unacceptable environmentally. Both methods, however, suffer from the fact that the concentrates containing either the salts or large amounts of solvents do not have either good dispersion stability, or more importantly, good storage stability. Typically, the partially swollen gum particles separate out of the formulation on standing either at room temperature or at elevated temperature, thus rendering the concentrates useless.
Another way of reducing the viscosity is obviously by the reduction of the polysaccharide gum level so that a manageable viscosity can be obtained. Reducing the gum level, however, compromises the fire fighting performance of the formulation because the amount of swollen polysaccharide is directly related to the thickness and therefore the effectiveness of the aforementioned membrane. Szonyi and Cambon in Fire Safety Journal, Vol. 16 (1990), 353-365, have recently developed a method of chemically modifying a polysaccharide gum, specifically a xanthan gum, using a fluorochemical which chemically reacts with the gum. They teach that this fluorinated xanthan gum is more effective than an unmodified gum. Therefore, it can be used at a lower level, which would lead to a lower viscosity in the formulated concentrate. However, they do not mention the extent of viscosity reduction.
In addition to the above methods, use of alkyl polyglycosides has been recently disclosed in U.S. Pat. No. 4,999,119 to help reduce the required amounts of polysaccharides and to thereby lower the viscosity of polar-solvent fire-fighting foam concentrates. We have now discovered that small amounts of certain anionic, water-soluble copolymers can greatly reduce the viscosity of viscous aqueous solutions of polysaccharide gums. As a further extension of this discovery, we have also discovered that the low viscosity polysaccharide gum solutions can be used in the formulation of extremely low viscosity polar-solvent fire-fighting foam concentrates, especially 3.times.3 formulations, without the aforementioned instability problems and without compromising fire-fighting performance. Up to a 40-fold reduction in viscosity was found possible with various 3.times.3 concentrates made with different types of polysaccharide gums. The anionic copolymers of were also found to significantly improve fire-fighting performance, especially on acetone fires.
No teachings have been found which describe the use of water-soluble, anionic copolymers to reduce the viscosity of either viscous polysaccharide gum solutions in water or in polar-solvent fire-fighting foam concentrates. However, these types of copolymers have been used in the past to accomplish the opposite, i.e. to increase the viscosity of various industrial formulations, as discussed further on.